But is any of this really true? Is it a case of feeding your assumptions into your research so you come up with exactly what you expected? If you define serious composition as always taking risks then you look into music history to find serious compositions and you come up with the Beethoven late quartets and they become the perfect example of what you were looking for and simultaneously are proof of your claim. Yes, they were "risky" compositions for several reasons: they experimented with form, they extended and expanded individual movements in some cases but in others added to the number of movements. They were risky and took a very long time to be accepted, but now, a sublime example of this risk, the C# minor quartet in seven movements, is in the repertoire of every string quartet and even in the small city where I live is heard at least once a year.
This raises a couple of questions: was Stravinsky correct when he said that this was music that would always be new or avant-garde? And what do those terms mean? When was modernism? What happens when the risky becomes the new ordinary?
But let's back up a bit and accept that our view of music history might just be shaped by our assumptions. The two ideas of emotional authenticity and progressive expansion of the musical "language" were both ideas characteristic of the 19th century. Theorists of that time supported the idea of breaking the "rules" of composition even as they were engaged in creating those very rules! Haydn, Mozart and Beethoven followed no rulebooks (true, they studied counterpoint from the traditional texts, but this was a training rather remote from what they actually did in their own work). There were no rules or formulas for writing sonata movements in the 18th century. These were all invented and written down in the mid-19th century, which is why few sonata movements by Haydn, Mozart and Beethoven actually conform to the rules. In an odd sort of historic reversal, the three Classical masters get points for breaking rules they were completely unaware of!
This is a process that seems perennial as we have young composers and performers today getting credit for breaking rules and going outside the box. It is a paradigm of the arts today.
But, and let me underline this, this is a paradigm that is no more than a hundred and fifty or so years old. Out of a written music history of a thousand years. Neither Haydn nor Mozart ever sat down to write a single piece wanting or hoping to break a rule or take a risk. Let me say that again: Neither Haydn nor Mozart ever sat down to write a single piece wanting or hoping to break a rule or take a risk. And I don't think Beethoven thought of what he was doing in those terms either. Remember, even in his day there were no rules for sonata form, so how could he take a risk by breaking them? Some of his most challenging compositions are challenging because of his intense use of counterpoint, which was, at his point in time, an archaic move and besides, one also pursued by Haydn and Mozart. Beethoven's intensity was often challenging, yes, to the point of perhaps risking the immediate approval of his audience and this was something greatly admired by the 19th century (or its composers, at least). In fact, this is largely the source of our basic assumptions regarding risk and serious composition. Beethoven is the paradigm.
So the corollary of this is that the music prior to Beethoven does not fit the paradigm. But still, music histories are written trying to project back the "serious composition is risk" model into the distant past. A composition like Bach's Art of Fugue, written due to his compelling urge to systematize and exhaust possibilities, is seen in our day as a bold work of avant-garde risk-taking. And on the reverse of the coin, the astounding breadth and depth of Haydn's music is consistently underrated because it plainly does not suit the paradigm. Neither does Mozart's, but in his case we have the romantic biography, pumped up by movies like Amadeus, to justify our enjoyment of his music despite its non-risky nature.
Here I want to speculate a bit: is it possible that this paradigm is wearing thin at last? After all the aberrant excesses of the 50s, 60s, 70s and on, are there any musical risks left to take? And is there any reason to take them? Once we have known total dissonance is there any reason to keep being dissonant? Is it interesting or not that a lot of young composers seem interested in consonance?
This trend seems particularly strong in the Baltic States. Here is a piece by the Latvian composer Pēteris Plakidis (the performance begins at the 1'40 mark):